The invention relates to a monolithic cascade cell for converting incident light radiation into electrical energy.
Many semi-conductive materials, particularly those of the Group III-V type, respond to incident photons by producing current flow and thus convert the incident light energy into electrical energy. Accordingly, such materials are attractive for conversion of solar energy into electrical energy.
There are a number of practical difficulties in utilizing such materials for generation of electrical power. One of the substantial difficulties is that a single solar cell of a given material forming either a homojunction or heterojunction is sensitive only to a limited range of photon energies. Photons above that range will not be efficiently converted into electrical energy since a portion of the energy above the range will be wasted as heat and photons below that range will not be converted at all, so that the resulting energy conversion efficiency of the cell is quite low, typically below 20%.
In order to increase that efficiency, it has been proposed to associate together a number of different semiconductive layers, each responsive to a different range of photon energies. The patent to Jackson, U.S. Pat. No. 2,949,498, suggests a stack of discrete solar cells of this type, each of which is capable of conversion of photons in a particular range of photon energy so that the cells together encompass a greater portion of the solar spectrum than a single cell.
However, stacked cells as taught by Jackson (i.e., cells which are fabricated separately and then mechanically attached to form a stack) are difficult and, therefore, costly to fabricate. The source of the difficulty arises because free carrier absorption within the cells substantially reduces the photon flux available to the lower bandgap cells in the stack when the higher bandgap cells are thicker than approximately 0.001 cm. (0.0004). Photons absorbed through free carrier optical transitions do not contribute to energy conversion and serve only to raise the temperature of the cell which in turn reduces conversion efficiency. In practice, therefore, the efficiency can be less than for a single cell. For high efficiency, the total thickness of each cell, above the bottom cell, must be considerably less than 0.001 cm. in order for the lower photon energy flux to pass unattenuated through each layer comprising a cell and into the next cell of lower bandgap. However, cell thicknesses of less than 0.010 cm. (0.004") cannot be reliably produced and assembled separately because of excessive cell breakage during processing and stack assembly.
In addition, to minimize photon flux shielding, the top and bottom contact pattern of each cell, except the bottom cell, should be identical and indexed from cell-to-cell during stack assembly. Alternatively, the bottom contact of each cell above the bottom cell may be of a peripheral pattern and indexed from cell-to-cell during stack assembly.
Moreover, the reflection losses, even with anti-reflection coatings or textured surfaces, at the top and the bottom surfaces of each cell in a stack also serve to reduce the flux available to cells further down in the stack. Reflection loss may be 15% at any one surface. Therefore, in a two-cell stack, this results in about a 40% drop in flux available to the bottom cell due to reflection losses alone.
The net effect of free carrier absorption and reflection losses on a voltage aiding, series connected, two-terminal stacked structure is that the efficiency may be substantially less than the efficiency obtained from any one of the cells operating as a single cell exposed to the full solar spectrum.
Many of the problems of a discrete stack can be overcome by a monolithic, cascade solar cell structure which circumvents the losses arising from free carrier absorption and multiple reflections. It also simplifies the electrical contact between cells. Because of the known technology available for fabricating monolithic, multiple layer devices, thin layers may be fabricated in which free carrier absorption loss is extremely small, and, therefore, its effect on efficiency, negligible. Due to the monolithic construction, reflection losses are also greatly reduced.
The patent of James, U.S. Pat. No. 4,017,332, describes a monolithic cascade solar cell structure in which some of these problems can be overcome by utilizing layers of different types of semi-conductive materials epitaxially deposited one on top of the other to eliminate problems of assembly and reflection. Thus, the efficiency of the overall cell can be increased provided that it is designed so that adverse factors as described generally above do not over-balance the potential increase in efficiency. In order to electrically connect the layers, and thus avoid the necessity for an intermediate terminal, a heterojunction is provided having dislocations due to lattice constant mismatch which, according to James, provides a low resistance series connection through tunnelling action between the two layers.
This lattice constant mismatch model is difficult to fabricate because it is difficult to insure that most or all of the dislocations reside at the interface which is a necessary condition for tunnelling action. Furthermore, the dislocations due to lattice mismatch will result in the dark current increasing in both the first and second layers, i.e., there will be an increase in the hole contribution of the first layer and in the electron contribution in the second layer. This will reduce the efficiency of the overall cell.
Another problem with the structure of James is that the layer (heterojunction) separating the potential generating layers is optically active, i.e., it has a bandgap lower than the top layer. This will generate a potential opposing the potentials generated in the top and bottom layers and reduce the efficiency, possibly even below the efficiency of one layer or cell alone.
The present invention relates to an improved cell of the general type described in the James patent in which the junctions within the first and second energy converting layers are homojunctions as in James but in which, in one embodiment, the third layer similarly provides a homojunction operating as a tunnel junction to connect the layers together. The use of a homojunction as in the present invention insures satisfactory tunnelling and a homojunction functioning as a tunnel diode can be satisfactorily produced with existing methods.
Further, according to the present invention, the bandgap of the third layer forming the tunnel junction is greater than the bandgap of the first layer so that the third layer is optically inactive and photons passing through that first layer are not converted into electrical energy in the layer which forms the tunnel junction, and thus pass unattenuated to the bottom or second layer. This is important because if this layer is optically active as in James, the generated photopotential will oppose the other two voltages and reduce the overall efficiency.
Applicant has discovered several specific configurations of solar cells using specific materials which provide high efficiency and are advantageous for a number of different reasons, including minimal lattice mismatch. When necessary, the effects of mismatch can be minimized by use of graded layers between the voltage producing layers. Efficiencies of 30-40% can be achieved. These specific embodiments are described in detail below.
Monolithic devices of this type can be made to have the generated voltages aid or oppose. For voltage aiding configurations only two terminals are required whereas at least three are required for a voltage opposing cell. For most applications, the two-terminal cell is preferred since it is simpler to fabricate. However, the three terminal configuration may exhibit a higher efficiency since the same terminal current need not flow through each junction. The two junction, voltage aiding cascade cell is highly attractive from the standpoint of both theoretical efficiency and high temperature performance. It is also likely to offer an improvement in radiation hardness over contemporary silicon cells.
According to a further embodiment of the invention described below, other types of detectors and light emitting diodes may be conveniently and advantageously fabricated in a cascade structure. In a specific illustrated embodiment, one layer is formed as a light emitting device and the other layer formed as a light receiving device operating at different wavelengths. Thus, a receiver-transmitter system can be configured within the same device, and the same optical fiber and/or optical system used to supply light to and take light from the device.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.